The long-range goals are to elucidate the molecular basis for the meiotic abnormalities that result in trisomy 21, and for the clinical phenotype that results from trisomy 21. The specific aims are to: 1) Examine features of the organization of DNA sequences about the centromere of chromosome 21, and their relation to meiotic abnormalities involving chromosome 21. 2) Isolate genes on chromosome 21 that encode surface proteins responsible for the increased adhesiveness of trisomy 21 fetal fibroblasts; these genes are potentially responsible for the congenital malformations associated with Down syndrome. The DNA studies will begin with a family of DNA sequences, the "724" family, that we localized to the pericentromeric region of all the human acrocentric chromosomes, including chromosome 21. An efficient recombination-based assay, which we continue to refine, will be used to "walk" to adjacent DNA sequences in chromosome 21-specific recombinant libraries. These studies will be conducted analogously to previous studies in which 724 and ribosomal DNA families were shown to be apposed within several thousand base pairs of each other in the human genome. Single copy DNA probes for the centromere of chromosome 21 will be isolated to study the parental origin of nondisjunction events, and to study the assortment of individual acrocentric centromeres in interphase and meiosis. The evolutionary basis for chromosome rearrangements involving the acrocentric chromosomes will be studied by localizing the 724 family in higher primates, as the apposition of 724 and ribosomal DNA sequences postdates the divergence of man from the New world owl monkey 35 million years ago. Our finding that trisomy-21 fetal lung and endocardial-cushion fibroblasts aggregate more rapidly in vitro than do normal control cells suggests that cell surface markers encoded by chromosome 21 may mediate this effect and thereby interfere with normal pulmonary and cardiac development in Down syndrome. Monoclonal antibodies that react with chromosome 21-encoded cell surface markers will be used to isolate genes on chromosome 21 that encode such markers. Particular attention will be focused on one such antibody that appears to inhibit the rapid aggregation of trisomy 21 fetal fibroblasts in vitro. Isolation of a gene that is responsible for the increased aggregation of trisomy 21 fetal fibroblasts could ultimately be used to study the genesis of congenital heart defects in trisomy 16 in mouse, and in trisomy 21 in man.